Computational studies of the effects of ortho‐ring and para‐ring activation on the kinetics of SNAr reactions of 1‐chloro‐2‐nitrobenzene and 1‐phenoxy‐2‐nitrobenzene with aniline

Computational studies are reported for reactions of 4‐substituted‐1‐chloro‐2,6‐dinitrobenzenes 1 , 6‐substituted‐1‐chloro‐2,4‐dinitrobenzenes 2 and some of the corresponding 1‐phenoxy derivatives 3 and 4 with aniline in the gas phase. The effects of substituent groups in the calculated energy values for reactants 1–4 , transition states structures, intermediates and products formed in the reactions between the compounds and anilines have been compared. Calculated bonds length and angles from optimized structures of the reactants were comparable with values reported for some of compounds 1–4 obtained by X‐ray crystal structures analysis. Generally, the decomposition of the Meisenheimer intermediate to the products requires more energy compared with the reactants except for when R = H. The order of stabilization of the intermediate was found to reflect the relative order of activation by substituents in the substrates. The 4‐substituted‐1‐chloro‐2,6‐dinitrobenzenes 1 and the phenoxy derivatives 3 were found to be more stable than their corresponding 6‐substituted analogues. This is an indication that the rate of nucleophilic attack at 1‐position will increase with increasing ring activation but may be reduced by steric repulsion at the reaction centre that increases in the order Cl < OPh. However, the steric hindrance to the steps involved in nucleophilic substitution by aniline is significantly increased when the substrates contain two ortho‐substituents. In most cases, the rate determining step is the decomposition of the σ‐adduct intermediate except with 1‐chloro‐2,6‐dinitrobenzenes 1 and 6‐substituted‐1‐chloro‐2,4‐dinitrobenzenes 2 , either because of reduction in ring activation or the presence of bulky ortho‐substituents in the chloro compounds 1 and 2 . Copyright © 2014 John Wiley & Sons, Ltd.     

Mechanism of nucleophilic substitutions at phenacyl bromides with pyridines. A computational study of intermediate and transition state

DFT computations have been performed on nucleophilic substitutions of phenacyl bromides with pyridines to investigate the mechanism of the reaction. In contrast with earlier suppositions, tetrahedral intermediate is not formed by the addition of pyridine on the C?O group of phenacyl bromide, because the total energy of the reacting species increases continuously, when the distance between the N and C(?O) atoms of reactants is shorter than 2.7 Å. At a greater distance, however, a bridged complex of the reactants is observed, in which the N atom of pyridine is slightly closer to the C atom of the C?O, than to the C atom of the CH₂Br group of phenacyl bromide, the distances are 2.87 and 3.05 Å, respectively. The attractive forces between the oppositely polarized N and C(?O) atoms in the complex decrease the free energy of activation of the S_N2 attack of pyridine at the CH₂Br group. The calculated structural parameters of the S_N2 transition states (TS) indicate, that earlier TSs are formed when the pyridine nucleophile bears electron‐donating (e‐d) groups, while electron‐withdrawing (e‐w) groups on phenacyl bromide substrate increase the tightness of the TS. Free energies of activation computed for the S_N2 substitution agree well with the data calculated from the results of kinetic experiments and correlate with the σ_Py substituent constants, derived for pyridines, and with the Hammett σ constants, when the substituents (4‐MeO‐4‐NO₂) are varied on the pyridine or on the phenacyl bromide reactants. Copyright © 2008 John Wiley & Sons, Ltd.     

Revision of the dual substituent parameter treatment using the DFT‐calculated reaction energies

The dual substituent parameter (DSP) data treatment is a broadly used procedure correlating the reaction energies or other physical quantities with two sets of substituent constants, inductive (σ_I) and resonance (σ_R). It was here revised using the most extensive sets of experimental reactivities available in the literature and two sets of reaction energies calculated at the level B3LYP/6‐311+G(d,p): acidities of 4‐substituted benzoic acids and 5‐(E)‐substituted penta‐2,4‐diene‐(E)‐acids with 19 or 15 common substituents. The latter two series enabled us to investigate the substituent effects more systematically than it was ever possible with the experimental data; this means in particular separate treatment of the undissociated acid molecules and of their anions, further separation of donor and acceptor substituents. In addition, the standard statistical treatment was improved when testing the significance of the resonance term. The DSP treatment is not valid generally, this applies both to the standard reference series and to the series commonly investigated. At best, DSP may be considered to hold for donor substituents but the effects of acceptors are much less variable and do not depend on the constants σ_R nor on any other measure of resonance. The small efficiency of acceptor substituents is due by the fact that the constant functional group (COOH in the standard series) is itself an acceptor. A correct treatment would be to investigate the donor and acceptor substituents separately, donors with an acceptor functional groups, and vice versa; substituents with weak resonance effect should be not included. The popularity and apparent success of the DSP treatment can be attributed to several grounds, most important has been the unbalanced choice of substituents. Copyright © 2007 John Wiley & Sons, Ltd.     

What can the geometry tell about the charge distribution in the mesoionic heterocycles? A DFT study on the SCN4R2 system

This computational organic chemistry study was performed using the hybrid functional B3LYP. Many basis sets were evaluated and the basis set 6‐31G(d) was found to be the most practical in terms of time and accuracy. The study presents the first method in the chemical literature that allows estimation of submolecular charges in mesoionic compounds. The theory was built on a reference model structure for which the absolute value of charge of the aromatic p‐orbitals is known. This is the cornerstone, and by employing the harmonic oscillator model of aromaticity (HOMA) many parameters can be quantified. The electronic structure of the title compound was the subject of this approach. The study addressed the following points: geometries, infrared frequencies, NMR chemical shifts, calculated charges, a chemical reactivity, and the frontier orbitals. The calculations illustrate that the π‐system of the CN₄ segment includes considerable aromatic character and carries a positive charge while the overall charge is negative. This allows classifying it as a σ‐acceptor/π‐donor while the exocyclic counter anion, the sulfur atom, is a σ‐donor/π‐acceptor. The substituents (R groups) in this case are only σ‐donors. The approach may be applied to other mesoionic and mesoaromatic systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Equilibrium adsorption of water-like molecules on single nanospheres

Water-like gas adsorption on spherical nano-aerosol particles has been studied for particles of different sizes at different temperatures with density functional theory. The water-like fluid was modelled as Lennard–Jones (LJ) spheres with four hydrogen-bonding sites with parameters adjusted to the phase diagram of water. For the single sphere case, both the adsorption excesses and density profiles approach those of the plane cases as the spherical substrate sizes increase; at each temperature studied, the size dependence of the transition from thin-film adsorption to thick-film adsorption has been observed. What we found here support earlier suggestions (J. X. Fang, W. H. Marlow, J. X. Lu, and R. R. Lucchese, J. Chem. Phys. 107, 5212 (1997)) that not only the interaction energies between the water molecule and the spherical substrate are sensitive to the size of the spherical substrate, but also the wetting behaviour. In calculations of the excess adsorptions of particles of radii R?=?20σ, 30σ, and 50σ, these substrates show the expected transition from the single molecule to the macroscopic aerosol particles.     

Density functional theory analysis of a mixed‐ligand iridium compound for multi‐color organic light‐emitting diodes

Electronic states and their energies are calculated for a mixed‐ligand Ir(III) compound, (5‐chloro‐8‐hydroxyquinoline) bis(2‐phenylpyridyl) iridium (called IrQ(ppy)₂‐5Cl) using time‐dependent density functional theory (TDDFT) calculations and are compared with the experimental result. A good agreement is obtained between the calculated and measured absorption spectra. The d‐π_Q* molecular orbital transition gives the lowest‐energy triplet state absorption band. Its energy is estimated as 1.84 eV (671 nm), which is close to the absorption band position of 1.86 eV (666 nm) observed for IrQ(ppy)₂‐5Cl doped in 4,4′‐N,N′‐dicarbazole‐biphenyl (CBP) host and of 1.88 eV (660 nm) observed for IrQ(ppy)₂‐5Cl doped in polystyrene (PS). The second triplet state absorption band is caused by d‐π_ppy transition. Its position is calculated as 2.51 eV (494 nm). The dipole moment is estimated as 3.45 D, which is lower than the dipole moment of fac‐Ir(ppy)₃. This is understood by a reduced charge transfer between Ir(III) and quinoline ligand. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanisms and Möbius strips: understanding dynamic processes in annulenes

Exploratory computational studies on annulenes with planar, Möbius, and two‐twist topologies have resulted in new mechanisms to explain facile thermal configuration change (cis‐trans isomerization) for medium‐sized annulenes ([12]‐ to [16]annulene). Möbius π‐bond shifting through both aromatic and antiaromatic transition states, two‐twist π‐bond shifting, and planar nondegenerate π‐bond shifting can all be invoked to explain experimental results. Moreover, a simple bond‐shift rule, which is based on the change in number of trans C?C double bonds (Δtrans), was developed that predicts the topology of the transition state(s) necessary to effect the desired cis‐trans isomerization. The bond‐shift rule was also applied to configuration change in dehydro[12]annulene. Finally, extensive investigation of the [14]annulene hypersurface revealed that numerous Möbius minima exist within 10 kcal/mol of the global minimum. Copyright © 2012 John Wiley & Sons, Ltd.     

Stereoelectronic effects in Menshutkin‐type SN2 reactions: theoretical study based on through‐space/bond orbital interaction analysis

Through‐space/bond orbital interaction analysis has been applied to investigate the stereoelectronic effects on stabilizing the transition state of Menshutkin‐type S_N2 reactions. The mechanism of how the substituent effects work on accelerating the reactions has been demonstrated from orbital interaction perspective. The geometrical structures and Mulliken charge distributions have been compared to elucidate the substituent effects for the S_N2 reaction center. It is found that the substituents lower the activation energies by strengthening the orbital interactions in the S_N2 reaction process. When electron‐donating and electron‐accepting substituents (–C₆H₅ and –CHO) are introduced to the same central carbon at the reaction center, the symmetry allows the π–π* interactions among the donor and acceptor in the transition state. It stabilizes the transition state much more than the reactant complex. And the π–π* interactions are estimated to decrease about 2.28 kcal/mol of the energy for transition state. The σ‐like orbitals of the partial bond around the central carbon are reactive, and the σ–π* orbital interactions stabilize the reactant complex much more than the π–σ* interaction. When the σ–π* and π–σ* interactions are deleted from the system, the activation energy increases and turns close to the values of the systems which are without such substituents. It can be concluded that the π–π*, σ–π*, and π–σ* interactions cooperatively accelerates the S_N2 reaction by stabilizing its transition state. Copyright © 2013 John Wiley & Sons, Ltd.     

Linear free‐energy relationships in semiquinone species and their Mn(II) and Cu(II) complexes

Linear free‐energy relationships for a series of functionalized semiquinone ligands and their Mn^II‐ and Cu^IIhydro‐tris(3‐cumenyl‐5‐methylpyrazolyl) borate complexes were examined. Quinone–semiquinone cycle half‐wave reduction potentials and semiquinone hydrogen hyperfine coupling constants (a_H) were determined and their correlation with Hammett σ parameters reported. A new σ parameter, σ_aH, has been proposed. Mn^II and Cu^II metal complex metal–ligand charge transfer and n → π* UV transitions were found to be modulated by substituents. Satisfactory Hammett correlations between UV transitions and various σ values have been determined and compared in a number of instances. Copyright © 2011 John Wiley & Sons, Ltd.     

The revision of intermolecular interactions in 1,3‐dinitrobenzene crystal—the role of nitro groups in optical nonlinearity

Polarized Fourier transform‐infrared (FT‐IR) reflectance spectra and powder Raman spectra have been measured for 1,3‐dinitrobenzene crystal in order to revise the assignments of bands by means of the oriented gas model reinforced with quantum chemical [density functional theory (DFT)] calculations. Longitudinal optical/transverse optical (LO‐TO) splitting of some bands is observed indicating medium strong, long‐range, dipole–dipole interactions. The analysis of overtones in the polarized FT‐NIR spectra has allowed us to estimate the anharmonicity of vibrations in the crystal. The molecular motions of the nitro groups are analyzed on the basis of temperature‐dependent polycrystalline IR spectra. Based on the values of the energy difference (Δν_el) between the forbidden A_1g→B_2u transition in the benzene molecule in the gas phase and the first electronic transition in 1,3‐dinitrobenzene, it has been concluded that the intermolecular interactions are medium strong. The nitro group interactions are proposed to play the main role in the optical nonlinearity. Copyright © 2010 John Wiley & Sons, Ltd.     

A combined experimental and density functional study of 1‐(arylsulfonyl)‐2‐R‐4‐chloro‐2‐butenes reactivity towards the allylic chlorine

Nucleophilic substitution and dehydrochlorination reactions of a number of the ring‐substituted 1‐(arylsulfonyl)‐2‐R‐4‐chloro‐2‐butenes are studied both experimentally and theoretically. The developed synthetic procedures are characterized by a general rapidity, cheapness, and simplicity providing moderate to high yields of 1‐arylsulfonyl 1,3‐butadienes (48–95%), 1‐(arylsulfonyl)‐2‐R‐4‐(N,N‐dialkylamino)‐2‐butenes (31–53%), 1‐(arylsulfonyl)‐2‐R‐2‐buten‐4‐ols (37–61%), and bis[4‐(arylsulfonyl)‐3‐R‐but‐2‐enyl]sulfides (40–70%). The density functional theory B3LYP/6‐311++G(2d,2p) calculations of the intermediate allylic cations in acetone revealed their high stability occurring from a resonance stabilization and hyperconjugation by the SO₂Ar group. The reactivity parameters estimated at the bond critical points of the diene/allylic moiety display a high correlation (R² > 0.97) with the Hammett (σ_p) constants. 1‐Arylsulfonyl 1,3‐butadienes are characterized by a partly broken π conjugated system, which follows from analysis of the two‐centered delocalization (δ) and localization (λ) index values. The highest occupied molecular orbital energies of 1‐arylsulfonyl 1,3‐butadienes are lower than those of 1,3‐butadiene explaining their low reactivity towards the Diels–Alder condensation. Copyright © 2015 John Wiley & Sons, Ltd.     

A DFT investigation into the structure and energetics for nonadiabatic proton transfer in the benzophenone/N,N‐dimethylaniline contact radical ion pair

For the past 60 years, the standard model for the interpretation of the mechanism for proton transfer has been based upon transition‐state theory, which posits that the transition state is found in the proton transfer coordinate involving the breaking and making of bonds. However, the observed dynamics of proton transfer within the triplet contact radical ion pair, derived from a variety of substituted benzophenones complexed with N,N‐dimethylaniline, cannot be accounted for within the standard model for proton transfer. Instead, the kinetic behavior is in accord with nonadiabatic proton transfer theory that has the transition state in the solvent coordinate. Evidence for the importance of the solvent coordinate comes from the existence of an inverted region; as the driving force for reaction increases, the rate of proton transfer decreases. This kinetic behavior is not found in the standard model. The present paper employs density function theory to examine the question as to whether the inverted region can be attributed to the transition state being in the solvent coordinate or whether the inverted region is an artifact produced by changes in the structure of the triplet contact radical ion pair with the placement of substituents upon the p,p′ positions of benzophenone. It is concluded that the inverted region is not an artifact of substituent effects upon structure. These results support the conclusion that the transition state for proton transfer resides in the solvent coordinate and challenges the validity of the standard model for interpreting the mechanism of proton transfer. Copyright © 2014 John Wiley & Sons, Ltd.     

Cation sitting in aromatic cages: ab initio computational studies on tetramethylammonium–(benzene)n (n=3–4) complexes

Quantum chemistry study was performed on interaction between tetramethylammonium (TMA) and aromatic cages by means of the MP2 method to show how TMA sits in an aromatic cage that is composed of benzenes. The MP2 calculations on TMA–(benzene)_n complexes demonstrate that the more the benzene molecules in the aromatic cage, the stronger the binding strength between the cage and TMA. In details, the structure of TMA–(benzene)_n (n = 1–4) complexes can be easily constructed by superimposing n TMA‐benzene complexes via TMA, and the binding energies of the TMA–(benzene)_n complexes are the sum of the n corresponding TMA‐benzene systems. For instance, the distances between the N of TMA and the plane of the benzene ring are 4.238, 4.252, 4.264 ,and 4.276 Å, respectively, for TMA–(benzene)_n (n = 1–4) complexes, and the BSSE corrected binding energies at MP2/6‐311++G** level are ?8.8, ?17.3, ?25.8 and ?34.3 kcal/mol, respectively, for TMA– (benzene)_n (n = 1–4) complexes. Thus, this study provides us useful information on how a cation interacts with an aromatic cage in terms of complex geometry and binding strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of neutral and charged substituents on the infrared carbonyl stretching frequencies in phenyl and alkyl benzoates in DMSO

The carbonyl infrared stretching frequencies for 57 meta‐, para‐ and ortho‐substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X and alkylbenzoates, C₆H₅CO₂R, containing besides neutral substituents the charged substituents in phenoxy and alkoxy part in dimethyl sulfoxide (DMSO) have been recorded. The carbonyl stretching frequencies, ν_CO, for meta‐ and para‐substituted phenyl esters of benzoic acids in the case of neutral substituents were found to correlate well with the substituent constants, σ°. The ν_CO values for ortho derivatives correlated with the inductive substituent constants, σ_I, only. The values of constants for charged substituents, σ°_±, calculated on the basis of the ν_CO and the ¹³C NMR chemical shifts, δ_CO, in DMSO agree well with the σ°_± values for the corresponding ion pairs reported by Hoefnagel and Wepster and those determined from the log k values of the alkaline hydrolysis in 4.4 M NaCl solution at 50 °C. Thus, the values of substituent constants for ion pairs of charged substituents estimated on the basis of aqueous data could be successfully used in non‐aqueous solution (DMSO) simultaneously with neutral substituents in case the charged substituents were not completely ionized and are in ion pair form. Copyright © 2016 John Wiley & Sons, Ltd.     

Kinetic study on the aromatic nucleophilic substitution reaction of 3,6‐dichloro‐1,2,4,5‐tetrazine by biothiols

The aromatic nucleophilic substitution reaction of 3,6‐dichloro‐1,2,4,5‐tetrazine (DCT) with a series of biothiols RSH: (cysteine, homocysteine, cysteinyl–glycine, N‐acetylcysteine, and glutathione) is subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the disappearance of DCT at 370 nm. In the case of an excess of N‐acetylcysteine and glutathione, clean pseudo first‐order rate constants (k_obs1) are found. However, for cysteine, homocysteine and cysteinyl–glycine, two consecutive reactions are observed. The first one is the nucleophilic aromatic substitution of the chlorine by the sulfhydryl group of these biothiols (RSH) and the second one is the intramolecular and intermolecular nucleophilic aromatic substitutions of their alkylthio with the amine group of RSH to give the di‐substituted compound. Therefore, in these cases, two pseudo first‐order rate constants (k_obs1 and k_obs2, respectively) are found under biothiol excess. Plots of k_obs1 versus free thiol concentration at constant pH are linear, with the slope (k_N) independent of pH (from 6.8 to 7.4). The kinetic data analysis (Brønsted‐type plot and activation parameters) is consistent with an addition–elimination mechanism with the nucleophilic attack as the rate‐determining step. Copyright © 2014 John Wiley & Sons, Ltd.     

Enantio‐differentiating reactions of a racemic γ‐lactone enolate with chiral esters. A DFT investigation

The acylation of lithium (±)‐spiro‐γ‐lactone enolate 5 by the O‐protected methyl (?)‐(S)‐lactate or the O‐protected methyl (+)‐(S)‐mandelate occurs through enantio‐differentiating reactions. The (S,S)‐enolate 5 is the most reactive with the lactate whereas the (R,R)‐enolate 5 selectively reacts with the mandelate. According to theoretical calculations at the B3LYP/6‐31G(++)(d,p) level of theory of 40 intermediates of this Claisen condensation, the experimental results are compatible with a previous chelation of the ester by an auxiliary cation lithium arising from the medium. The addition reaction occurs through a chelation process mediated by the counterion of the enolate. More stable tetrahedral intermediates including two lithium cations result from an antiperiplanar transition state. These results clearly demonstrate that the presence of a second lithium cation (the first lithium cation is solvated by di‐isopropylamine and the second one is solvated by a THF molecule or a di‐isopropylamide anion) stabilizes the tetrahedral intermediate and is compatible with an antiperiplanar transition state according to the Felkin–Anh model. Copyright © 2010 John Wiley & Sons, Ltd.     

External heavy atom effect on the triplet state of aromatic hydrocarbons: IV. Fluorene,triphenylene-d12 and coronene

The phosphorescence decay functions and spectra of ethanol solutions of triphenylene-d₁₂, coronene and fluorene were observed in the presence of different KI concentrations at 77 K. The analysis of the dependence of phosphorescence (T₁ → S₀) rate constant, k_PT, and intersystem crossing (T₁ ? S₀) rate constant, k_GT, on triplet energies E_T of aromatic molecules and on the distance between the heavy atom and the aromatic molecule show that the external heavy atom effects on k_PT and k_GT are essentially of the second and third order, respectively. The phosphorescence spectra of triphenylene and coronene show the presence of the third order contributions in the radiative transition, which involves the second triplet state of aromatic molecule.     

Recognition of aromatic amino acids and proteins with p‐sulfonatocalix[4]arene – A luminescence and theoretical approach

The host–guest interaction of p‐sulfonatocalix[4]arene (p‐SC4) with aromatic amino acids (AAs) and two proteins has been studied using UV–Vis absorption, fluorescence, and theoretical methods. Spectral studies supported by binding constant and calculated binding energy (BE) values show that p‐SC4 binds more strongly with tyrosine compared with other AAs. The application of Bader's theory of atoms in molecule shows the involvement of various types of noncovalent interactions in the formation of the host–guest complexes. Both tyrosine and histidine have strong electrostatic interaction with the sulfonato group and other two AAs have dominant π?π interaction with the aromatic rings of calixarene. In addition, the role of C?H···O, C?H···π and lone pair···π (lp···π) interactions in the stabilization of p‐SC4‐AA complexes has also been realized from the atoms in molecule analysis. The electron density at the bond critical points varies with the calculated BEs and trend in BEs is in good agreement with the experimental binding constant values. The work has been extended to the binding of p‐SC4 with proteins, bovine serum albumin and ovalbumin. Ovalbumin exhibits stronger binding with p‐SC4 than bovine serum albumin. Copyright © 2012 John Wiley & Sons, Ltd.     

Kinetics,mechanism and thermodynamics of reactions of CH3NHNH2 with OOH

Reactions between CH₃NHNH₂ and OOH radical were studied using computational methods. The activation energies (E_a) and Gibbs free energies of activation (ΔG^#) were calculated at the MP2 and B3LYP levels of theory. The calculated activation energies of the hydrogen abstraction reactions were less than 100 kJ/mol and those for the substitution reactions were about 150–250 kJ/mol. The calculated activation energies for the intra-molecular hydrogen transfer reactions in CH₃NHNH, CH₂NNH₂ and CH₃NN molecules were 210–250 kJ/mol. Catalytic effect of the water molecule on the intra-molecular hydrogen transfer reactions was studied. It was found that the water molecule decreases the activation energies by about 70–100 kJ/mol. Rate constants of the reactions were calculated using transition state theory in the temperature range of 298–2000 K. Consecutive hydrogen abstraction reactions from CH₃NHNH₂ led to the formation of CH₂NN, which was a very stable molecule.     

A study on a primitive artificial esterase model: reactivity of a calix[4]resorcinarene bearing carboxyl groups

The host molecule octacarboxymethyl calix[4]resorcinarene 1 catalyses the hydrolysis of substituted phenyl N‐methylpyridinium‐4‐carboxylate esters 3a–f by complexation followed by intracomplex reaction via an anhydride intermediate. The reactivity in the presence of 1 is higher than that of the background at low pH; at high pH an inversion of reactivity occurs, the background becomes predominant since the host inhibits hydrolysis of the esters. The reactivity of esters 3a–f complexed with the host suffers little change in effective charge on the phenolic oxygen (?0.15 units) in contrast with the changes observed in alkaline hydrolysis (?0.28 units) and in the hydrolysis of the model monoaryl glutarate esters (‐1.02 units). The less negative effective charge in the transition state for host 1 catalysis compared with that in the glutarate case is ascribed to stronger solvation by water molecules in the complex compared with that due to water molecules in the bulk solvent.